1. Field of the Invention
This invention relates to a flat panel display control system of a portable computer capable of selectively displaying image data supplied from two or more display controllers on the flat panel display apparatus.
2. Description of the Related Art
A conventional lap-top type computer comprises a built-in display controller 1 and a RAMDAC 3 as well as a flat panel display unit 5 such as a liquid crystal display (LCD) unit for visual monitoring, as illustrated in FIG. 25. The image data transmitted from the display controller 1 are converted to video signals by the RAMDAC 3 and fed to the flat panel display unit 5 where they are displayed on the display screen.
Normally, a lap-top type computer is so designed that it can be optionally connected to a color CRT display unit. FIG. 26 illustrates a block diagram of a lap-top computer having a color liquid crystal display unit as a standard component and connectable to a CRT display unit as an optional component. In FIG. 26, a system bus 7 comprises a 24-bit width address bus and a 16-bit width data bus. A video RAM (VRAM) 11 stores display data to be displayed in color on a color CRT display unit 13 or a color LCD panel 15. The VRAM 11 has a memory capacity of 64K.times.16 bits. A CPU 9 writes the display data in the VRAM 11 by way of a CRT controller 17.
The CRT controller 17 reads the display data from the VRAM 11 and sends the read display data to a display control section 21 and a display data generating section 23 via a display data bus 19.
The display data generating section 23 includes a CRT palette 25A and a Digital-Analog Converter (DAC) 25B. The CRT palette 25A converts the display data transmitted from the CRT controller 17 to color data. The DAC 25B converts the color data into R, G, B analog video signals and feeds them to the color CRT display unit 13. The CRT display unit 13 displays the R, G, B video signals in synchronism with the horizontal synchronizing signal (HSYNC) and the vertical synchronizing signal (VSYNC) 27 sent from the CRT controller 17.
The display control section (DC) 21 comprises: an arithmetic circuit 29 for obtaining gradation parameters of 39 gradations by using given arithmetic formulas and palette data stored in the CRT palette 25A in response to updating of the data stored in the CRT palette 25A; a conversion table 31 for converting gradation parameters into gradation data of 16 gradations; and an LCD palette 33 into which the gradation data generated by the conversion table 31 are set. The LCD palette 33 converts the display data fed from the CRT controller 17 into R, G, B and I data of 4 bits. The operations of such an arithmetic circuit 29 and a conversion table 31 is described in detail, for instance, in U.S. patent application Ser. No. 07/406,066.
The color LCD panel 15 displays an image defined by the R, G, B and I data fed from the digital port of the display control section 21 in synchronism with the HSYNC and VSYNC signals.
Data bus 35 is used for transferring data between the system bus 7 and the display control section 21. The data bus 37 is used for writing color data into the CRT palette 25A.
VGAs (Video Graphics Arrays) have been used as a graphics subsystem of portable computers. A VGA is capable of displaying in high resolution images with 640.times.480 pixels and 250 colors on a CRT display unit. Meanwhile, versatile graphical user interfaces (GUI) such as windows and OS/2 presentation managers (PM) have also been developed.
To work such advanced GUI features effectively, it is desirable to use a high resolution graphics subsystem. And also, to open a plurality of windows on a single screen, a high resolution screen is also necessary.
In an attempt to meet such demands in a usual portable computer, there has been developed a portable computer that can be connected with an external display board through a Feature connector 46, as shown in FIG. 27. The display board includes a graphics subsystem and has an ability to display, on a CRT display unit, an image of higher resolution than that displayed by the graphics subsystem incorporated in the computer.
In FIG. 27, a color CRT display unit 49 connected to a CRT connector 47 which is provided on the computer main body displays data fed from a display controller 51 provided in the computer main body. A CRT display unit 55 connected to a CRT connector 53 provided on a display board, which is plugged into an expansion slot of the portable computer, displays data transmitted from the display controllers 51 and 52 in an alternative manner.
Known conventional portable computers to which optional display board can be connected can use only CRT display units as display units.
As the lap-top type or notebook-type portable computer technologically advances and gains popularity, there has arisen a demand for portable computers having a flat panel display unit that can selectively display different image data from a number of graphics subsystems.
Further, in the conventional computer, it is impossible to supply image data from the display controller 52 to the CRT display unit 49 connected to the standard CRT connector 47.